1. Technical Field
The present invention relates to power amplifiers and, more particularly, to a power control circuit and technique for automatically controlling the output power of an amplifier over a wide dynamic range.
2. Discussion
Power amplifiers are well known electronic devices used for various applications including radio transmit/receive communication systems. Power amplifiers employed for radio communications are generally referred to as radio frequency (RF) power amplifiers. For radio communications, it is generally desirable to employ an RF source which provides a plurality of constant power output levels. Techniques have been devised for providing automatic power control of such amplifiers to maintain a constant power output among a plurality of power settings. As an example, the cellular radio transmitter operates at one of eight assigned output power settings which are generally separated 4 dB from one another. In addition, it is usually required that the cellular radio transmitter maintain its output power within a +2/-4 dB tolerance.
Conventional techniques for providing automatic power control of the output of an amplifier generally involve measuring the output power of the amplifier with a diode power detector connected directly to the output of the amplifier. This connection is typically made with a passive device such as a small capacitor or a directional coupler, both of which provide a sample of the amplifier's output power. The sampled power is rectified by a detecting diode in the power detector to provide a DC voltage signal which has a magnitude that is proportional to the output power of the amplifier. The DC voltage signal is conventionally compared with a setpoint voltage in a comparator circuit to provide an error voltage. The resulting error voltage is then used to adjust the gain of the amplifier so as to obtain the desired amplifier power output.
Such conventional power control techniques generally have a limited reliable detection range due to the narrow amplitude range of the detecting diode. The detection range is usually bounded at the high end by diode saturation, and at the low end by diode temperature dependency characteristics. Diode saturation occurs when an increase in the amplifier's power no longer produces a corresponding increase in the detected diode voltage, resulting in an error in the power reading. In an attempt to avoid this problem, one type of conventional technique includes boosting the saturation point of the diode detector by increasing the bias current to the diode. However, this technique still suffers from the diode temperature effect at the low end power.
Typical power control techniques may generally include a diode power detector which employs a schottky barrier type of detector diode. Generally, diodes such as the schottky barrier type incur a temperature induced error when the voltage detected is relatively small, such as on the order of tens of millivolts. In particular, the schottky barrier type of diode exhibits a silicon semiconductor junction characteristic which is temperature dependent at a rate of approximately two millivolts per degree centigrade. At higher operating power, the voltage across the diode is relatively large as compared with the temperature induced error so that the temperature error factor becomes somewhat insignificant.
A second conventional power control technique employs a second diode having characteristics which precisely match that of the detector diode. This technique operates to counteract the temperature effect in the diode detector. The second diode does not operate as a power detector but merely operates to compensate for the temperature error of the detecting diode. Generally, the second diode's temperature induced voltage is then subtracted out of the detector diode's voltage to provide a corrected power reading. However, this method which involves a closely matched diode pair is relatively expensive and often requires that the diode pair be hand selected for purposes of providing proper matching. In addition, this conventional technique does not offset the diode saturation effects as discussed earlier.
Another prior art method utilizes a logarithmic amplifier for compressing the dynamic range of the power signal received by the power detector. As a result, the logarithmic amplifier narrows the dynamic range that the detector has to cover. However, a logarithmic amplifier is generally a relatively complicated and expensive electronic device. In addition, the logarithmic amplifier's gain typically varies considerably with temperature. Furthermore, the logarithmic amplifier compresses the detection dynamic range so that the detected voltage range is relatively small in comparison to the large power control range. As a result, this causes the control loop to become highly sensitive such that control instability may result therefrom.
It is therefore desirable to obtain a power control technique for automatically controlling the power output of an amplifier which is not limited to the dynamic range of a power detector. More particularly, it is desirable to obtain such a technique that extends the upper and lower limits of a diode power detector's operating range without suffering from the prior art limitations.